A vacuum pump (ON/OFF-Mode) is controlled by means of switches that change its switching point with the changing ambient pressure, for example as a result of the altitude above sea level changing. As a result, as the altitude increases the absolute OFF switching pressure which is controlled by the switch decreases, and consequently from a certain altitude it is no longer possible for the vacuum pump to go below the predefined absolute pressure in order to switch off the pump. This means that starting from this time the pump can no longer switch off. The critical operating altitude for this fault event is the lowest the absolute pressure which is defined for switching off the pump. If the pump reaches a minimum absolute pressure of 14% of the ambient pressure and if the absolute pressure for switching off the pump is 233 mbar at sea level, the pump can no longer switch off starting from an altitude of approximately 1000 m. This leads to unnecessarily long pump running times and service life problems.
Possible pump damage can be avoided by limiting the running time of the pump in combination with an ON/OFF hysteresis avoidance algorithm. However, this compromise increases pump running time.
Pumps may be actuated by fixed switching thresholds, wherein the control pressures do not change with the altitude. Fixed values relating to the altitude for the switching on and switching off signals are possible only with sensors. Fixed pump switching thresholds result in a significant reduction in the differential pressure as the operating altitude increases. As a result, the available deceleration potential decreases at the modulation point of the vacuum brake booster. Starting at a critical operating altitude, the switching on threshold decreases to zero bar, and the pump can no longer be switched on that leads to a failure of the vacuum supply.
EP 1 311 418 B1 describes a method for monitoring a vacuum apparatus which detects defects in the suction pump or leakages in the system. An initial pressure is measured, and after a time interval a final pressure in the vacuum accumulator is measured, and the difference between the pressures is compared with a limiting value that can also be determined as a function of the ambient pressure. A signal is generated when the limiting value is undershot.
One exemplary object of the invention is to improve the control of a vacuum apparatus. Additional advantages of the invention are defined in the dependent claims.
According to a first aspect of the invention, a method for controlling a vacuum apparatus of a pneumatically operated servo unit of a motor vehicle, in which vacuum is applied to a vacuum accumulator via a pump, has the following steps:                determining the pressure in the vacuum accumulator;        determining the ambient pressure;        adapting pump control signals to the ambient pressure;        comparing the pressure in the vacuum accumulator with the adapted pump control signals;        outputting a pump actuation signal as a function of the comparison.        
The steps do not necessarily have to be carried out in the specified order. Therefore, for example, the ambient pressure can be determined first.
The ambient pressure to be determined may be already available in an engine controller. The value for the ambient pressure for controlling the vacuum apparatus can be transmitted, for example, via a vehicle-mounted CAN bus.